Dearomatization-aromatization

Schmalz has reported on the intramolecular radical cyclization of (arene)Cr(CO)3 complexes (Scheme 10). Whilst most of the products from these reactions were transformed into aromatics, dearomatized products can potentially be accessed via this new and appealing approach [32-34]. 

Deprotonation of a pyridinylmethylenic proton of pyridine- and bipyridine-based pincer complexes can lead to dearomatization. The dearomatized complexes can then activate a chemical bond (H-Y, Y = H, OH, OR, NH2, NR2, C) by cooperation between the metal and the ligand, thereby regaining aromatization (Figure 1.1). The overall process does not involve a change in the metal s oxidation state [6-8]. In this chapter, we describe the novel, environmentally benign catalytic synthesis of esters, amides, and peptides that operate via this new metal-ligand cooperation based on aromatization-dearomatization processes. 

Figure 1.1 Metal-ligand cooperation based on aromatization-dearomatization.

Metal-Ligand Cooperation Based on Aromatization/Dearomatization Processes 101... 

Zell T, Milstein D. Hydrogenation and dehydrogenation iron pincer catalysts capable of metal—ligand cooperation by aromatization/dearomatization. Acc Chem Res. 

Like in the case of 2, the bipyridine-based Ru(II)-PNN pincer complex 6 plausibly displays a novel type of metal-ligand cooperative activity through aromatization-dearomatization processes. Recently, a DFT calculation on classical C-O cleavage vs our newly reported C-N bond breaking in the amide hydrogenation reaction was reported by Cantillo [69]. 

Gunanathan C, Milstein D (2011) Bond activation by metal-ligand coopruation Design of green catalytic reactions based on aromatization-dearomatization of pincer complexes. Top Organomet Chem 37 55... 

Gunanathan C, Milstein D (2011) Metal-ligand cooperation by aromatization-dearomatization a new paradigm in bond activation and great catalysis. Ace Chem Res 44 588... 

Bond Activation by Metal-Ligand Cooperation Design of Green Catalytic Reactions Based on Aromatization-Dearomatization of Pincer Complexes... 

Ru(II) hydride complexes based on electron-rich PNP and PNN ligands of the type depicted on Fig. 13 (which can undergo aromatization/ dearomatization steps) efficiently and selectively catalyze the acceptorless dehydrogenation of primary alcohols to esters and H2 with high TONs... 

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